Boot Up Process and BIOS explained

Layer #	Layer
0	Hardware
1	BIOS
2	Operating System
3	Applications

The BIOS software lives on a ROM (Read Only Memory) Circuit on the motherboard known as a Complementary Metal Oxide Semiconductor(CMOS). People often incorrectly refer to the BIOS setup utility as CMOS, however, CMOS is the name of the physical location that the BIOS settings are stored in.

BIOS routines are the first thing that begin when the computer is turned on. They are made up of 3 main operations:

First, the Power On Self Tests (POST) are conducted. These tests verify that the system is operating correctly and will display an error message and/or output a series of beeps known as beep codes depending on the BIOS manufacturer.

Second, is initialization in which the BIOS looks for the video card. In particular, it looks for the video card's built in BIOS program and runs it. The BIOS then looks for other devices' ROMs to see if any of them have BIOSes and they are executed as well.

Third, is to initiate the boot process. The BIOS looks for boot information that is contained in file called the master boot record (MBR) at the first sector on the disk. If it is searching a floppy disk, it looks at the same address on the floppy disk for a volume boot sector. Once an acceptable boot record is found the operating system is loaded which takes over control of the computer. The order that the BIOS looks for devices can be determined in your BIOS settings. Usually it searches in this order CD Drive, Floppy, C Drive.